fix(core): CFN version and description template sections were merged …

…incorrectly

In the merge logic in Stack when rendering the template,
it was mistakenly assumed that all CFN sections are objects.
However, there are some sections, like Description and AWSTemplateFormatVersion,
that are in fact strings.
Add special logic for those cases in the merge functionality
(multiple provided CFN versions are checked for being identical,
and mutliple descriptions are merged together, with a newline in between).

Fixes aws#8151

packages/@aws-cdk/core/lib/stack.ts

@@ -848,12 +848,32 @@ function merge(template: any, part: any) {
     if (!dest) {
       template[section] = dest = src;
     } else {
-      // add all entities from source section to destination section
-      for (const id of Object.keys(src)) {
-        if (id in dest) {
-          throw new Error(`section '${section}' already contains '${id}'`);
+      if (typeof dest === 'object') {
+        // add all entities from source section to destination section
+        for (const id of Object.keys(src)) {
+          if (id in dest) {
+            throw new Error(`section '${section}' already contains '${id}'`);
+          }
+          dest[id] = src[id];
+        }
+      } else {
+        // the sections are strings, like Description and AWSTemplateFormatVersion
+        // treat each differently
+        switch (section) {
+          case 'Description':
+            // merge the two descriptions, with a newline in between
+            template[section] = `${dest}\n${src}`;
+            break;
+          case 'AWSTemplateFormatVersion':
+            // check if the version are the same, and error out if not
+            if (dest !== src) {
+              throw new Error(`Conflicting template versions provided: '${dest}' and '${src}'`);
+            }
+            break;
+          default:
+            // let's error out by default - we can add other special cases later
+            throw new Error(`Duplicate section '${section}'`);
         }
-        dest[id] = src[id];
       }
     }
   }

packages/@aws-cdk/core/test/test.include.ts

@@ -50,6 +50,30 @@ export = {
     test.throws(() => toCloudFormation(stack));
     test.done();
   },
+
+  'correctly merges template sections that contain strings'(test: Test) {
+    const stack = new Stack();
+
+    new CfnInclude(stack, 'T1', {
+      template: {
+        AWSTemplateFormatVersion: '2010-09-09',
+        Description: 'Test 1',
+      },
+    });
+    new CfnInclude(stack, 'T2', {
+      template: {
+        AWSTemplateFormatVersion: '2010-09-09',
+        Description: 'Test 2',
+      },
+    });
+
+    test.deepEqual(toCloudFormation(stack), {
+      AWSTemplateFormatVersion: '2010-09-09',
+      Description: 'Test 1\nTest 2',
+    });
+
+    test.done();
+  },
 };
 
 const template = {